THE 90-K PLATEAU OF OXYGEN DEFICIENT YBA2CU3O7-DELTA SINGLE-CRYSTALS

Low field magnetization, resistivity, and specific heat measurements were performed on large, high-quality YBa2Cu3O7-delta single crystals in the range 0.05<delta<0.20, the so-called 90 K plateau. For each crystal the measurements were repeated in several states with a different oxygen concentration. Using the normal state resistivity as a relative measure for the oxygen concentration, it is demonstrated that with increasing delta, T(c) goes through a small but well defined maximum near delta=0.12 whereas the jump in the specific heat at T(c), DELTA-C(p)/T(c), dramatically drops by a factor of three between delta=0.10 and 0.20.